Maxum used Perko deck fills (gas, water, waste) and they are all the same size. Replace every 5 years is what they recommend. Perko o-rings are available at all marine shops.
You can also get much cheaper ones at a local automotive shop by taking in the old one and matching it up as Mike suggested.
I just did all mine this year and it's a good feeling to have that done and out of the way.